How Digital Light Processing (DLP) Works

The term "3D printing" refers to building up an object additively. This is accomplished using a number of different methods. Digital Light Processing is one type of 3D printing process.

Overview

igital Light Processing DLP process

Digital Light Processing uses a projector, like the kind used for office presentations or in home theaters, to project the image of the cross section of an object into a vat of photopolymer (light reactive plastic). The light selectively hardens only the area specified in that image. The most recently printed layer is then repositioned to leave room for unhardened photopolymer to fill the newly created space between the print and the projector. Repeating this process builds up the object one layer at a time. DLP is known for its high resolution, typically able to reach a layer thicknesses of under 30 microns, a fraction of a sheet of copy paper. Like Stereolithography (SLA), objects printed using DLP are known to have less visible layers than are visible with other techniques, such as Fused Deposition Modeling (FDM), at the same resolution.

Materials

DLP printers can produce objects with a wide variety of properties such as water resistance, flexibility, durability, stiffness, high clarity, thermal resistance and high impact resistance. The photopolymers have been designed to mimic ABS, polypropylene, and wax, making them useful for everything from high quality prototyping to lost wax casting. However, prints using photopolymer can become brittle with increased exposure to light over time. They may begin to show small cracks and become more susceptible to breaking. With this process, only one material can be used at a time because the object is built out of a vat containing a singular photopolymer solution.

How it works

A digital micromirror device (DMD) is the core component of DLP printers. The DMD projects a light pattern of each cross-sectional slice of the object through an imaging lens and onto the photopolymer resin. The projected light causes the resin to harden and form the corresponding layer which fuses it to the adjacent layer of the model. Compared with Stereolithography (SLA), DLP can have relatively faster build speeds. This is because a single layer is created in one digital image, as opposed to SLA’s laser process which must scan the vat with a single point. SLA can be likened to drawing the layer one motion at a time while DLP is more akin to a stamping process.



There are two styles of printing using DLP. The model can be built by pulling the object out of the resin to create space for uncured resin at the bottom of the tank to form the next layer. The other technique to print using DLP is to build the object by pulling it down into the tank with the newest layer being cured on the top of the photopolymer bath. Because of this, for all instances of sharp angles or overhangs, there must always be vertical supports connecting them to the build platform. Supports are thicker in their columns and base than where they connect to the object, to withstand the tensions and stresses of mechanical movement; for example, when disconnecting the most recently cured layer from the vat to create room for new resin to flow in. However, the points of contact between the support and the model can be minimized, resulting in an ability to have strong support columns while maintaining easy removal and post processing later, as well as minimizing surface blemishing.



With DLP, layers tend not to form ridges, blending together much more smoothly than plastic filament is able to. When the process is complete, the remaining resin must be washed away with solution, then the supports are removed by snapping or cutting. Sanding or filing away what’s left behind by the supports is usually left until later, after the model has had a chance to fully harden, which can be expedited with a short time under UV lamps.

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